1. Field of the Invention:
This invention relates to a novel spherical fine particulate pigment which is readily dispersed, possesses a large tinting strength, and provides a distinct color (a color of high saturation). The invention also relates to a process for preparing such a spherical fine particulate pigment.
2. Description of the Background
Conventionally, lake dyes are widely used because of their distinctness of color, availability of variety of colors, and their safety. Such lake dyes comprise a water soluble dye such as, for example, an acidic dye, and an insolubilizing agent thereof such as, for example, aluminum hydroxide carrying on its particle surface said water soluble dye absorbed. But, the aluminum lake, for example, has only a low intensity of color, and suffers a problem of color migration from the lake especially for an aqueous electrolyte solution. In order to improve the safety of dyes, there is proposed a technique for preparing a pigment by reacting a naturally occurring coloring material with a basic aluminum (Japanese Patent Laid-open No. 71128/1978). This method can provide a water insoluble pigment, but the pigment particles are large and irregular in shape. The dyes also has problems with dispersing ability and tinting strength.
The present inventors previously had found a method for improving the dissolution resistance of an aluminum lake by the use of a basic aluminum chloride, and filed a patent application (Japanese Patent Application No. 160323/1986). An aluminum lake had, however, a problem of poor tinting strength, as its dyeing concentration is only 10 to 40%. A trial has been made to render the dye insoluble by the use of a suitable precipitating agent. But, this method does not provide a satisfactory effect depending on the dye's structure. Also, the present inventors prepared a pigment by reacting a basic polyaluminum salt with a dye. But, the pigment obtained had irregularly shaped particles which tended to agglomerate to form a block with a diameter of 1.mu. to some fifteen microns, and did not provide a desired tinting strength.
It is generally known that the smaller the pigment's particle, the larger the tinting strength of the pigment. For this reason various methods are proposed for producing pigments with fine particles. One example is a method comprising first preparing a pigment with specific comminution assisting agents added thereto, and subsequently pulverizing the pigment particles (Japanese Patent Laid-open Nos. 58061/1984 and 100168/1984). But, the pigments prepared by this method are not uniform in particle shape and size, and thus are not satisfactory in both dispersing ability and tinting strength. Another method is to prepare a powdery pigment dispersant, which comprises dispersing a pigment in a w/o type emulsion resin solution, precipitating the pigment in water, and then drying it (Japanese Patent Laid-open No. 63936/1977). The pigment prepared by this method, however, is comminuted particles of resinous agglomerates which are not uniform in shape and size, and can provide only a poor tinting strength because of its resin- composite structure. Still another method heretofore proposed is to prepare a bead-like pigment suspension by block polymerization of a pigment and a polymeric monomer, and then further effecting suspension polymerization or emulsion polymerization (Japanese Patent Laid-open No. 8033/1977). The method can provide a pigment of spherically shaped particles, but the pigment uses a resin as a carrier and the diameter of the particles ranges from 1.0.mu. to tens of microns so that it does not provide a satisfactory tinting strength.
In order to improve the tinting strength of a dye, it is desirable to insolubilize the dye active ingredient by means of a suitable precipitating agent. But, a conventional insolubilizing reaction only provide irregularly shaped particles which are often agglomerated. Comminution of the agglomerated pigments also provides irregularly shaped and non-uniform particles with poor dispersing ability and tinting strength.
Pigment particles are often surface-treated to improve their dispersing ability, as the smaller the size of the particles the less dispersed they become. But, particles with poor uniformity can not be easily dispersed in the treating solution, thus making it difficult to surface-treat them.
As mentioned above there has been no pigment whose particles are uniform both in shape and size, and which provides satisfactory dispersing ability and tinting strength.
The present inventors have made extensive and earnest studies in order to solve the above problems in conventional pigments, and as a result found that a pigment comprising fine and spherical particles with diameters of less than 0.5.mu. can be prepared by reacting a water soluble dye and its insolubilizing agent in a w/o type emulsion water phase, and further, that treatment of the pigment particle surface with a surface active agent can provide a pigment with excellent dispersing ability and tinting strength. Such findings have led to the completion of this invention.